This invention relates in general to testing apparatus for electronic devices such as integrated circuits ("IC's"). More specifically, it relates to an electrical contactor assembly that is generally frequency insensitive to allow broadband testing with fast rising signals and is readily assembled and disassembled to facilitate field servicing and provide significant advantages in cost of manufacture.
In the manufacture and use of IC's packaged as surface mounted devices ("SMD's"), it is important to test the devices accurately, reliably, and at a high rate. Automatic testing and handling machines that can perform this task are available. Such apparatus suitable for testing SMD ICs are sold by the Daymarc Corporation, Waltham, Mass., under the trade designation Model 757. In one common type of SMD device, a PLCC packaged IC, the circuit is contained in a molded plastic body having a generally square or rectangular, box-like configuration. SMDs typically include either two rows of contacting leads along opposite and parallel sides of the body, or four rows of contacting leads, one along each side of the body. The most common configurations of SMDs include four rows of connecting leads. In any event, the leads lie generally in a common connecting plane.
An SMD is designed to be mounted directly on the surface of a circuit board or within a suitable receiving socket. The SMD can be distinguished from dual-in-line packaged (DIP) integrated circuits in that DIP devices are intended for mounting with leads passing through the circuit board (or within a suitable socket) rather than for surface mounting. Additionally, DIP's typically include only two rows of parallel connecting leads, in contrast to the usual four-row SMD.
Prior art SMD testing apparatus can generally be described as either manual or automatic. In a manual apparatus, an operator manually places each SMD into a socket, conducts the test, then removes the SMD. In addition to being an obviously slow and time-consuming procedure, the sockets tend to wear out rapidly. A typical life is only a few thousand devices. Replacing a socket requires de-soldering the old one and installing a new socket.
A manual testing apparatus, however, using a socket mounted directly the test circuit board is advantageous in that testing is done in the electro-magnetic environment of actual use. While automatic apparatus have proven to be fast, they test the SMD at a remote location from the test circuit, and thus in an inappropriate environment. As an example of the importance of proximity to the test circuit, it is known that merely changing the lead length in the test situation from the actual use situation by a quarter of an inch can lead to substantial changes in electrical response.
Another design consideration for contactors is that SMD leads tend to be very soft or delicate. A force component as small as a few grams in a direction parallel to the plane of the SMD can damage the leads. A single lead damaged in testing can render the entire SMD unsuitable for use. In several known automatic testing apparatus, the testing contacts exert a side-acting force which permanently displaces the lead causing lateral or longitudinal row misalignment. Other prior art devices use sloped surfaces to guide the SMD into test position. Depending on factors such as the extent of misalignment, such guidance mechanisms can also misalign the leads.
Still another design consideration, as described in U.S. Pat. No. 4,473,798 to Cedrone et al, is that the testing of integrated circuits frequently requires that the test signal be "fast-rising", i.e., a signal which is a very steep, step-like increase in potential. A typical fast-rising signal may be characterized by a voltage change of 1 volt per nanosecond. Such a signal can be represented through Fourier Series analysis as being composed of a multitude of superimposed sine waves having a very high frequency, typically on the order of 300 MHz. The fast-rising signal launched by the test circuitry and carried by the contacts to the device therefore behave in the manner of a high frequency signal.
With such high frequency "components" in the signal, the inherent inductance of the contacts themselves becomes a problem. Inductive reactance X.sub.L produces distortions and reflections which degrade the quality and accuracy of the test. The inductance L of the contact is a function of the cross-sectional configuration of the conductor and its length. Inductance increases directly with the length and inversely as a function of cross-sectional width. Since the inductive reactance X.sub.L =2.pi.fL, for the very high frequencies f associated with a fast-rising signal, the inductive reactance associated with even the relatively short contacts in normal use becomes a significant source of distortion and limits the accuracy of measurements.
One possible solution would be to increase the width of the contacts. However, the physical constraints of the test environment limit the available dimensions of the contacts. For example, the contacts must be separated laterally from adjacent contacts while each still maintaining a unique association with one lead on the SMD. Another possible solution is to make the contacts shorter. This is difficult to execute in testing DIP ICs, and in testing SMDs while the contacts can be made short compared to those in DIP contactors, the signal path from the contacts to the test circuit is long enough to affect signal integrity adversely.
Still another possible solution is simply to test each device more slowly to wait for distortions and reflections to die out. With many modern SMDs such as large gate arrays, however, the speed of operation of the device itself is so fast that if the testing operation were to extend over a sufficient period of time to allow distortions and echoes induced y the fast-rising testing signal to subside, then the speed rating of the devices could not be determined. In short, the testing operation must have a speed on the order of the device function being tested.
Another consideration is minimizing "ground noise", that is, changes in the reference voltage due to current surges during the test procedure simulating operation of the device. A typical situation is a test where a change in the device state causes a current surge in the range of 20 milliamperes per nanosecond. Such a surge can cause the ground reference to move one volt or more thereby distorting measurements referenced to ground by 20% or more. The end result is that good devices may not pass the test and are downgraded. The use of "surge" capacitors to provide power de-coupling is known, but there is no system which meets this problem while also meeting all of the other design problems noted herein.
Yet another problem with prior contactors is that in general the contacts are individually soldered to signal transmitting wires which in turn are soldered to contacts that connect to the test circuit. This mode of assembly involves a comparatively high cost of manufacture and it is not conducive to field repair or modification to accommodate devices with varying numbers of pins. Also, prior devices for testing SMD IC's have not offered a simple arrangement to connect a ground plane (or other arrangement for producing a distributed capacitance) to the ground of the test circuit, or one where the connecting arrangement can accommodate variations in the lateral spacing between the contactor assembly and the site of connection to the circuit board ground.
It is therefore a principal object of the invention to provide a contactor assembly that maintains signal integrity when testing SMD devices, even when the testing involves fast-rising signals, where the contactor is formed of a few readily aligned and assembled components to facilitate field servicing and modification of the contactor to accommodate different devices and different test circuits.
Another principal object is to provide a contactor assembly with the foregoing advantages which also includes a simple and highly effective arrangement for ground de-coupling the DUT or, more generally, linking selected pins through an electronic device situated in extremely close physical proximity to the DUT.
Another principal object of the invention is to provide a simple, adjustable connection between a ground plane associated with the contacts and the ground of the test circuit.
Another object is to provide the foregoing advantages while also providing a short signal path between the DUT and the test circuit.
Another object is to provide the foregoing advantages while also providing an electro-magnetic testing environment that closely simulates that of the intended use.
Yet another object of the invention is to avoid harm to the delicate SMD leads.
Still another object of the invention is to provide a virtual ground for selected leads close to the SMD.
A further object is to provide a contactor with the foregoing advantages that has a favorable cost of manufacture as compared to prior devices with comparable performance characteristics.